Autoimmunity has long been recognized as a feature of cancer, but only recently have data provided strong evidence that a humoral response is targeted against proteins that are mutated or overexpressed in tumor cells. It is presently unclear if the repertoire of autoantigens recognized by cancer patients will be unique to each individual, or will include a subset of common antigens. However, given that there are a finite number of ways by which cells may become cancerous, we hypothesize that the humoral response in cancer may frequently include recognition of proteins that are related either directly or indirectly to the process of tumorigenesis. Identification of common autoantigens may be useful for developing tools for early diagnosis and outcome prediction. We propose to take a two-pronged approach to identify such antigens. We will screen 1300 breast cancer patient sera on immunoblots of nuclear antigens to identify common reactivities. Sera with high titre autoantibodies to selected antigens will be used to clone corresponding cDNAs from expression libraries. In addition, we will identify messenger RNAs that are commonly overexpressed in breast tumor relative to normal breast by microchip expression profiling. Corresponding cDNAs will be cloned and expressed, and proteins used in specific ELISAs to test for recognition by breast cancer patient sera. Associations will be sought between serum reactivities, patient diagnosis and existing ten-year outcome data to assess the potential diagnostic and prognostic value of each antigen.